ABSTRACT Magnetic resonance imaging (MRI) provides exquisite spatial resolution, spectral sensitivity, and a rich variety of contrast mechanisms for diagnostic medical applications. Nuclear imaging using gamma cameras offers the benefits of using small quantities of radioactive tracers that seek specific targets of interest within the body. We have developed a new imaging and spectroscopic modality, recently published in the journal Nature, which utilizes favorable aspects of both approaches. Spatial information is encoded into the spin orientations of tiny amounts of a polarized radioactive tracer using pulses of both radio-frequency electromagnetic radiation (RF) and magnetic-field gradients, as in MRI. Rather than detecting the inherently weak electromagnetic signals from the precessing magnetization, however, imaging information is obtained through the detection of gamma rays emitted from the polarized nuclei. Unlike nuclear imaging, even a single gamma-ray detector can be used to acquire an image; no gamma camera is needed. Our new modality takes advantage of the fact that polarized nuclei with spin > emit gamma rays in a spatially anisotropic fashion with respect to the direction along which they are oriented. We refer to our new technique as Polarized Nuclear Imaging (PNI). Despite our successful proof-of-concept demonstration, significant challenges remain in moving to in vivo applications. The long-term goal of our research is to establish PNI as a practical medical imaging technology that addresses unmet medical diagnostic needs. Our short-term goal addressed in this application is to demonstrate the feasibility of in vivo use, by producing the first polarized nuclear images in a living animal. Our specific aims are: (1) to develop a pulse sequence strategy for PNI that is suitable for in-vivo application, and to demonstrate its efficacy by acquiring two-dimensional images in glass-cell phantoms; and (2) to demonstrate polarized nuclear imaging for the first time in a living animal, by acquiring one-dimensional images of inhaled 131mXe in rabbit lungs. Successful completion of these aims will lay the basis for PNI to become a practical new imaging modality, and will demonstrate the potential of PNI to create a new class of medical tracers.